Location and relative density of receptors. Importantly, different receptor types predominate in different tissues. For example, although both α and β receptors are expressed by pancreatic islet beta cells, the predominant response to a sympathetic discharge is mediated by α2 receptors.
Physiological Actions  of Adrenomedullary Catecholamines
Because the adrenal medulla is directly innervated by the autonomic nervous system, adrenomedullary responses are very rapid. Furthermore, because of the involvement of several centers in the central nervous system (CNS), most notably the cerebral cortex, adrenomedullary responses can precede onset of the actual stress (i.e., they can be anticipated) (see
Fig. 43.5 ). In many cases the adrenomedullary output, which is primarily epinephrine, is coordinated with sympathetic nervous activity as determined by the release of norepinephrine from postganglionic sympathetic neurons. However, some stimuli (e.g., hypoglycemia) evoke a stronger adrenomedullary than sympathetic nervous response and vice versa.
Many organs and tissues are affected by a sympathoadrenal response (
Table 43.2). An informative example of the major physiological roles of catecholamines is the sympathoadrenal response to exercise. Exercise is similar to the “fight-or-flight” response but without the subjective element of fear, and it involves a greater adrenomedullary response (i.e., endocrine role of epinephrine) than a sympathetic nervous response (i.e., neurotransmitter role of norepinephrine). The overall goal of the sympathoadrenal system during exercise is to meet the increased energy demands of skeletal and cardiac muscle while maintaining sufficient oxygen and glucose supply to the brain. The response to exercise includes the following major physiological actions of epinephrine (
Fig. 43.6): 1. Increased blood flow to muscles is achieved by the integrated action of norepinephrine and epinephrine on the heart, veins and lymphatics, and nonmuscular (e.g., splanchnic) and muscular arteriolar beds.
•Fig. 43.4 Stepsinsynthesisandsecretionofcatecholaminesfromadrenalmedullarychromaffincells.
2. Epinephrine promotes glycogenolysis in muscle. Exercising muscle can also utilize free fatty acids (FFAs), and epinephrine and norepinephrine promote lipolysis in adipose tissue. Epinephrine increases blood glucose by increasing hepatic glycogenolysis and gluconeogenesis. The promotion of lipolysis in adipose tissue is also coordinated with an epinephrine-induced increase in hepatic ketogenesis. Finally, the effects of catecholamines on metabolism are reinforced by the fact that they stimulate glucagon secretion (β2 receptors) and inhibit insulin secretion (α2 receptors). Efficient production of adenosine triphosphate (ATP) during normal exercise •Fig. 43.5 Stimulithatenhancecatecholaminesecretion.
cAMP,cyclicadenosinemonophosphate;DAG,diacylglycerol.
Arteriolardilation:↓ BP↑ Arteriolarvasoconstriction; (β2)(muscle)↑ BP(α1)(splanchnic,renal,cutaneous,genital)
BP,bloodpressure.
(i.e., a 1-hour workout) also requires efficient exchange of gases with an adequate supply of oxygen to exercising muscle. Catecholamines promote this by relaxation of bronchiolar smooth muscle.
3. Catecholamines decrease energy demand by visceral smooth muscle. In general a sympathoadrenal response decreases overall motility of the smooth muscle in the gastrointestinal (GI) and urinary tracts, thereby conserving energy where it is not immediately needed.
Metabolism of Catecholamines